Organic light emitting device incorporating a waveguide

ABSTRACT

The present invention provides an organic light emitting device having a first electrode and a transparent electrode with an organic light emitting layer therebetween; characterized by a waveguide provided on the opposite side of the transparent electrode compared to the organic light emitting layer. The present invention also provides a device incorporating at least two such organic light emitting devices so as to provide a pulsed modulation output or a multi-color output.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an organic light emitting devices andother devices incorporating the same.

2. Description of Related Art

Light emitting devices are well known and have been used in manyapplications. In simple terms electric signals are used to stimulatelight emissions and the emission of light from organic materials isassociated with electronic transitions between energy levels.

An example of a basic device is shown in FIG. 1. The devices arenormally structured as two electrodes (2,4) sandwiching a photoemittinglayer (6) and provided on a transparent substrate (8). The electrodesare used to cause the movement of electrons and holes into thephotoemitting layer and the recombination of the electron/hole pairs inthe photoemitting layer results in light emissions.

Typically, the transparent substrate (8) is formed from glass, silica ora plastic. The upper, cathode electrode (2) is typically formed fromCa/Al or Li/Al or LiF/Al. The lower electrode (4) must be transparentand is typically formed from ITO (Indium Tin Oxide), ZnO₂, Indium ZincOxide or GaN.

The photoemitting layer (6) may preferably be formed from an organicmaterial. The organic material may be of a low molecular type asdescribed in the Article by Q W Tang in Applied Physics Letters 1987pages 913 to 915. Alternatively, the organic layer may be of a polymertype as described in the Article by J H Burroughes in Nature 347 onpages 539 onwards published in 1990. Examples of low molecular typematerials are shown in FIG. 2. Examples of polymers include polyfluoreneand some of the derivatives thereof and these are shown in FIG. 3.

Light emitting devices using an organic layer (OLED) can be used incolour displays since the colour of the emitted light can bepredetermined by selecting the particular chemical structure of theorganic layer.

One of the advantages of organic materials is that they are relativelyeasy to handle during manufacture. For example, low molecular types maybe applied by sublimation, e.g. vaporisation, whereas polymers may beapplied by spin coating. During the subsequent processing of the device,a number of further features will need to be patterned. Such patterningusing photo lithography often degrades the light emitting efficiency ofthe device. The impact of patterning can be reduced when using organicmaterials since low molecular types enables metal masks to be used andpolymer types enables ink jet deposition techniques to be used. The easeof manufacturing the organic material itself and the improved resultingperformance of the device following subsequent patterning, also enableslarge size devices to be formed.

As discussed previously, LEDs function by stimulating the injection ofelectrons and holes into the photoemitting layer. FIG. 4 illustrates inmore detail the injection of electrons and holes and the recombinationof electron/hole pairs, resulting in the generation of light emissions.FIG. 4 illustrates in 1) charge being injected, 2) charge beingtransported, 3) charges being captured to form an exciton and 4)electron/hole pairs recombining. In order to maximise the lightemissions, the energy levels between the materials of the electrodes (2,4) and the photoemitting layer (6) need to be carefully selected. Theenergy levels need to be selected such that there is an appropriateenergy level gradient from the transparent electrode (4) to the cathode(2).

To improve the efficiency of OLEDs conjugated polymers may be usedinstead of a single layer of organic material. Conjugated polymersprovide an interface within the photoemitting layer such thatelectron/hole pair recombinations tend to become concentrated at theinterface. This leads to a concentration of light emissions from theimmediate vicinity of the interface and reduces the possibility ofdirect transmission of electrons or holes across the photoemittinglayer. The polymers shown in FIG. 3 are particularly suited to use in aconjugated polymer organic layer in an LED.

Waveguides enable light to be transmitted more efficiently from a sourceto a desired point of application.

Waveguides also form an essential part of the structure of semiconductorlasers, an example of which is shown schematically in FIG. 5. Thearrangement comprises an inorganic photoemitting layer (12) disposed ina confinement layer (11). On each side of the confinement layer is acladding layer (10), each of which is supported by a substrate (2).Light propogates in the photoemitting layer (12) and the confinementlayer (11) and is reflected by the mirrors (14) (one of which issemi-transparent) such that a narrow, highly concentrated beam ofcoherent light is output by the laser.

In the semiconductor laser, the electrodes are not of course made oftransparent materials.

Faced with a desire to concentrate the light output from an OLED, itwould seem natural to consider the use of a waveguide. In addition, thestructure of semiconductor lasers would lead to consideration of an OLEDin which both electrodes are optically opaque and in which a waveguideis arranged to be “in-plane” with the light emitting layer. However, anumber of difficulties arise. Importantly, organic materials tend tohave a low conductivity. Therefore, in practical devices, the layerneeds to be relatively thin, often in the order of 200 nm for many ofthe polymers. If the photoemitting layer is this thin, the output of acorresponding in-plane waveguide would exhibit a diffraction effectwhich would lead to poor coupling efficiency of the device with otheroptical devices. If the thickness of the layer is increased, then due tothe poor conductivity of the organic material, the driving voltage tendsto become prohibitively high.

SUMMARY OF THE INVENTION

Against this background, the present invention provides an organic lightemitting device comprising: a first electrode and a transparentelectrode with an organic light emitting layer therebetween;characterised by comprising a waveguide provided on the opposite side ofthe transparent electrode compared with the organic light emittinglayer.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way offurther example only and with reference to the accompanying drawings, inwhich:

FIG. 1 is a schematic diagram of a prior art OLED;

FIG. 2 gives examples of low molecular type materials;

FIG. 3 gives examples of polymers typically used for forming conjugatedpolymers;

FIGS. 4a) and b) are schematic diagrams of the energy levelconsiderations in an OLED;

FIG. 5 is a schematic diagram of the the structure of a semiconductorlaser;

FIG. 6 is a schematic diagram of a preferred embodiment of the presentinvention;

FIG. 7 is a schematic diagram and associated mathematical relationshipsof the photoemitting layer, transparent electrode and the waveguide;

FIGS. 8a) and b) are schematic diagrams of a further embodiment of thepresent invention;

FIG. 8c) is similar to FIG. 7 but shows the inclusion of a claddinglayer;

FIG. 9 is a schematic diagram of another embodiment of the presentinvention;

FIG. 10 is a schematic diagram of the present invention used to providecolour displays or high speed modulation pulsed outputs;

FIG. 11 is a timing diagram illustrating the formation of a high speedpulsed output;

FIG. 12 is a schematic diagram of a number of OLEDs according to thepresent invention arranged on a single substrate; and

FIG. 13 illustrates the relationship between wavelength, refractiveindex and absorption constant for examples of conjugate polymers, namelypolyfluorene and three derivatives thereof.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention provides an OLED with an external waveguide, forexample as shown schematically in FIG. 6. The device includes awaveguide (16) arranged between the transparent electrode (4) and asubstrate (18).

Preferably, the substrate (18) is of a transparent material. Generally,the light emitted from an OLED is viewed in a direction perpendicular tothe photoemitting layer. In the embodiment of the present inventionillustrated in FIG. 6, the materials of the cathode (2), the transparentelectrode (4) and the waveguide (16) are selected carefully inconjunction with the material of the photoemitting layer (6) so as toenable light to be emitted from the photoemitting layer at an angle andto enhance entry into the waveguide (16). The materials are selected inaccordance with their refractive indices and FIG. 7 demonstrates thebasic principles involved.

FIGS. 6 and 7 show simple embodiments and such arrangements have thedisadvantage of absorption by the cathode electrode. The insertion of abuffer layer between the cathode and the photoemitting layer mitigatesthis disadvantage. Light is completely reflected by the dielectricboundary when sinθ₁ is greater than n₃/n₁.

In the embodiments of FIGS. 6 and 7, when the substrate is of an opaquematerial, absorption by the substrate could also decrease the intensityof light. The buffer layer can enable the use of an opaque substratewithout absorption of light thereby.

The equations given in FIG. 7 define the desirable relationship betweenwavelength, refractive indicies and thickness of the photoemitting layerand the transparent layer. The thickness of the photoemitting layer canbe determined from other parameters.

FIG. 8 illustrates a further embodiment of the present invention. Inorder to maximise the efficiency of the OLED, a buffer layer (20) may beprovided between the waveguide (16) and the substrate (18). The bufferlayer assists in reducing the loss of light to the substrate. Typicallythe buffer may be formed from SiO₂, polymer (photo polymerisation resin)or polyamide. If a buffer layer is used, then it is possible toeffectively decouple the substrate so that the optical characteristicsthereof do not significantly influence the functioning of the waveguide.

The OLED may be further enhanced by the provision of a cladding layer(22) between the cathode (2) and the photoemitting layer (6). Thecladding layer reduces the loss of light to the cathode (2). Thecladding layer may be formed from methyl-substituted poly (p-phenylene)or one of the polyfluorene derivatives.

FIG. 8c is similar to FIG. 7 but shows the inclusion of a cladding layer(22). In this figure n₃ is the refractive index of the cladding layerrather than the refractive index of the cathode, as in FIG. 7. In FIG. 7the cathode has a large absorption and thus n₃ in that arrangement is acomplex number. In the arrangement shown in FIG. 8c, the cladding layeris transparent and n₃ is therefore a real number. Again, the equationsgiven define the desirable relationship between wavelength, refractiveindicies and thickness of the photoemitting layer and the transparentlayer. But, the thickness of the photoemitting layer can be determinedfrom other parameters.

If the refractive indices of the various components are designated asfollows:

Nsub Substrate (18) Nbuf Buffer (20) Nwav Waveguide (16) NtrnTransparent electrode (4) Nemt Photoemitting layer (6) Ncld Claddinglayer (22)

Then the desired relationships between the refractive indices are:

Nsub or Nbuf<Nwav

Nwav<Nemt and Ncld<Nwav

Ncld<Ntrn

Typically, the values for the refractive indices are in the followingranges:

Nsub Opaque Substrate (18) Nsub Transparent Substrate (8) 1.45-1.6  NbufBuffer (20) 1.4-1.5 Nwav Waveguide (16) 1.6-2.2 Ntrn Transparentelectrode (4) 1.6-2.0 Nemt Photoemitting layer (6) 1.7-2.7 Ncld Claddinglayer (22) 1.5-1.8

Instead of being restricted to the particular selection of materialsdependent upon the relative refractive indices, a Distributed BraggReflector (DBR) may be used between the waveguide (16) and the substrate(18); with or without the presence of a buffer (20),. FIG. 9 illustratesthe use of a DBR (24) arranged between the waveguide (16) and thesubstrate (18). The DBR comprises an arrangement of alternating layersof material of low refractive index with high refractive index. If thelow refractive index layers have a thickness d_(L) and a refractiveindex of n_(L), and the high refractive index layers have a thicknessd_(H) and a refractive index of n_(H), the layers will be selected asfollows:

(d _(L) n _(L) +d _(H) n)cos θ=λ/2 where λ is the wavelength of light.

In these circumstances the DBR (24) provides for very high reflectivityand wavelength selectivity.

The present invention as discussed above in relation to the variousembodiments may be used in a wide variety of applications due to theadvantages associated with the present invention. For example, since thephotoemitting layer can be retained as a relatively thin layer, a lowdriving voltage can be used which leads to a low power consumption,there is less thermal effect and less deteriation of the materials usedtherein. The thick waveguide provides a high intensity output with asmall divergence angle in the output beam. This facilitates efficientcoupling to other optical devices such as lenses, fibre optics and thelike.

Any one of the aforegoing embodiments of the present invention may beused in various applications. For example, FIG. 10 illustrates a numberof OLEDs (26) arranged on a single waveguide. In the device shown, thereare four OLEDs 26(1), 26(2), 26(3) and 26(4) each emitting light intothe same waveguide.

Each OLED may be used to generate light of a different wavelength. Ifthe OLEDs (26) are arranged to generate light, at least one of whichgenerates red, blue or yellow, then the device will be capable ofproviding full multi-colour displays.

Alternatively, as shown in FIG. 11 each OLED may be driven to emit apulse of light so as enable the device to provide high speed modulationpulsed outputs. If the first OLED 26(1) emits pulse I1 and the next OLED26(2) emits pulse I2 etc, then the emerging beam will have themodulation shown at the bottom of the Figure. This has the advantage ofreducing the duty cycle of the individual OLEDs, which results in lessheat generation and other improvements associated therewith.

A number of OLEDs may also be arranged each with their own respectivewaveguide and fabricated on the same substrate, as shown in FIG. 12. Theuse of OLEDs in the type of arrangement shown in FIG. 12 can achievesignificantly larger sizes than a similar arrangement of semiconductorlasers (which are limited to a single substrate wafer size typically of10-15 cm). The illustrated arrangement has particular application to theuse in printers, photocopiers, portable facsimile machines and the like.In such machines, a photo-conductive drum, photo reactive material orfilm is illuminated by the linear array of OLEDs. A driving systemstimulates each of the OLEDs to generate the desired output forilluminating with the photo-conductive layer. Uniformity of theindividual light sources is significantly improved since the whole arrayis fabricated as a single unit.

Reference is now made to FIG. 13 which shows the relationships betweenwavelength, refractive index and absorption constant for an example of apolymer, namely polyfluorene and a derivative; which materials can beused in a conjugated polymer arrangement. On review of these graphs, itcan be seen that for certain ranges of wavelength, eg 520 nm to 780 nm,the absorption constant is relatively low yet the refractive indexremains high. Thus, at a wavelength within this range the production andtransmission of light in arrangements according to the present inventioncan be optimised using these materials.

Arrangements according to the present invention may beneficially employa polymer such as polyfluorene or one of its derivatives as the materialof the waveguide. Thus, the light efficiency from such an OLED can befurther optimised.

The aforegoing description has been given by way of example only and itwill be appreciated by persons skilled in the art that modifications canbe made without departing from the scope of the present invention.

What is claimed is:
 1. An organic light emitting device, comprising: afirst electrode; a transparent electrode; an organic light emittinglayer disposed between the first electrode and the transparentelectrode; and a waveguide provided on the opposite side of thetransparent electrode compared to the organic light emitting layer andhaving an interface with the transparent electrode, wherein at least oneof a refractive index and a thickness of the light emitting layer and arefractive index and a thickness of the transparent electrode arearranged to cause, in operation of the device, resonance of lightemitted by the light emitting layer within the light emitting layer andthe transparent electrode and to cause light emitted from the lightemitting layer to enter the waveguide at a shallow grazing anglerelative to the interface between the waveguide and the transparentelectrode.
 2. An organic light emitting device as claimed in claim 1,further comprising a substrate on the opposite side of the waveguidecompared to the transparent electrode.
 3. An organic light emittingdevice as claimed in claim 2, wherein said substrate is opticallyopaque.
 4. An organic light emitting device as claimed in claim 2,further comprising a buffer layer disposed between the waveguide and thesubstrate.
 5. An organic light emitting device as claimed in claim 1,further comprising a cladding layer disposed between the first electrodeand the organic light emitting layer.
 6. An organic light emittingdevice as claimed in claim 1, in which the refractive index of thewaveguide is less than the refractive index of the light emitting layer.7. An organic light emitting device as claimed in claim 1, furthercomprising a Distributed Bragg Reflector disposed on the opposite sideof the waveguide compared to the transparent electrode.
 8. An organiclight emitting device as claimed in claim 1, in which the waveguide isformed of a conjugated polymer.
 9. A device comprising at least twoorganic light emitting devices as claimed in claim 1, wherein respectivewaveguides of the at least two organic light emitting devices areprovided in the form of a single waveguide common to the at least twoorganic light emitting devices.
 10. A device as claimed in claim 9,comprising a driving circuit for driving the organic light emittinglayer of each light emitting device to generate light at different timesfrom each other.
 11. A device as claimed in claim 9, wherein the organiclight emitting layer of each light emitting device is selected to emitlight with a respectively different wavelength distribution.
 12. Adevice as claimed in claim 9, comprising three light emitting deviceseach arranged to emit a respective one of red, blue and yellow light.13. A device as claimed in claim 9, wherein the at least two organiclight emitting devices are formed on a common substrate.
 14. An organiclight emitting device as claimed in claim 1, wherein at least one of therefractive index and the thickness of the light emitting layer, and therefractive index and the thickness of the transparent electrode arearranged according to the following equation:${\frac{4{\pi \left( {{n_{1}d_{1}{Cos}\quad \theta_{1}} + {n_{2}d_{2}{Cos}\quad \theta_{2}}} \right)}}{\lambda} - {2\varphi_{S}}} = {2\quad m\quad \pi}$

where m is a whole integer; n₁ is the refractive index of the lightemitting layer; d₁ is the thickness of the light emitting layer; n₂ isthe refractive index of the transparent electrode; d₂ is the thicknessof the transparent electrode; φ_(S) is the phase shift which occurs atan interface between the light emitting layer and the first electrode;θ₁ is the angle of incidence of the light emitted by the light emittinglayer, and propagating in the light emitting layer, relative to aninterface between the light emitting layer and the transparentelectrode; θ₂ is the angle of incidence of the light emitted by thelight emitting layer, and propagating in the transparent electrode,relative to the interface between the transparent electrode and thewaveguide; and λ is the wavelength of the light emitted by the lightemitting layer in operation of the light emitting device and resonatingwithin the light emitting layer and the transparent electrode.
 15. Anorganic light emitting device as claimed in claim 1, wherein the shallowgrazing angle is 10° or less.